1. Field of the Invention
The present invention relates to an exhaust emission purifying apparatus for an internal combustion engine, and in particular, to an exhaust emission purifying apparatus adopted for use in purifying nitrogen oxides (NOx) in the exhaust gas from an internal combustion engine by adding a reducing agent for the NOx to the exhaust gas through an injection nozzle disposed in an exhaust passage.
2. Description of the Related Art
There has been known the following apparatus as an apparatus for purifying nitrogen oxides, or NOx, contained in the exhaust gas of an on-vehicle engine such as a diesel engine. Namely, in this apparatus, a reduction catalyst is disposed in an exhaust passage of the engine, and also, an injection nozzle for a reducing agent is disposed on the upstream of this reduction catalyst in the exhaust passage. In such an apparatus, the reducing agent is added to the exhaust gas by means of the injection nozzle, and NOx in the exhaust gas and the added reducing agent are reacted with each other on the reduction catalyst, so that NOx can be purified. As the reducing agent, other than the ammonia water, the urea water and another liquid reducing agent are used. The reducing agent is stored in a storage tank in a liquid state, and a necessary amount of reducing agent is injected by the injection nozzle. An injection amount of the injection nozzle is controlled according to engine operating conditions, so that the reducing agent can be added in just proportion, and NOx can be efficiently purified (refer to Japanese Unexamined Patent Publication No. 2000-027627).
However, the above described exhaust emission purifying apparatus for an engine has the following problems.
Firstly, in this exhaust emission purifying apparatus, the injection nozzle is fixed to a pipe wall upper portion of the exhaust passage, and also, a nozzle hole thereof is set to open toward a forward direction to the flow of the exhaust gas, and also to direct downward in a vertical direction. Therefore, the injected reducing agent is carried on the flow of the exhaust gas, and is difficult to sufficiently mix with the exhaust gas, and thus the reaction on the reduction catalyst does not proceed sufficiently so that the desired exhaust emission purifying performance cannot be achieved. Further, since the nozzle hole is set to direct downward, the injected reducing agent is readily to be attached to a pipe wall bottom face of the exhaust passage, and a reducing agent amount which reaches the reduction catalyst tends to be short. To increase an injection pressure of the reducing agent in order to accelerate the mixing of the reducing agent with the exhaust gas is not necessarily preferable, since there are problems in the excessive adding of the reducing agent and the running cost of the apparatus. In the case where the reducing agent is excessively added due to the increase of the injection pressure, such a problem occurs that all of the added reducing agent might not be consumed in the reaction with NOx, and a partial reducing agent passes through the reduction catalyst to be discharged into the atmosphere.
Secondly, in this exhaust emission purifying apparatus, since the injection nozzle is disposed in a manner hanging from the pipe wall upper portion of the exhaust passage, the reducing agent is susceptible to the dripping after the injection. If the deposition of the reducing agent occurs in the nozzle hole and at the periphery thereof due to this dripping, an amount of the reducing agent to be added lacks due to the clogging, resulting in the degradation of the exhaust emission purifying performance.
Thirdly, in this exhaust emission purifying apparatus, in order to start an engine operation under such a low temperature environment where the reducing agent is frozen, a heater is disposed to a storage tank so that the frozen reducing agent is defrosted by this heater. However, since this storage tank has a relatively large volume, a considerable amount of time is needed until the reducing agent is defrosted to enable the injection. Therefore, the sufficient exhaust emission purifying performance cannot be achieved immediately after the engine operation is started.